Newly discovered fossils of Castrocauda lutrasimilis, a beaver-like animal, suggests that mammals swam with dinosaurs during the Jurassic era.

"Its lifestyle was probably very similar to the modern day platypus," Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, said in a statement. "It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest."